This invention relates to the diffusion bonding of metals, such as aluminum and titanium, and their alloys, and which are subsequently formed into structures such as truss core panels by superplastic forming, and is particularly concerned with the elimination of core distortion of core panel structures following superplastic forming, particularly when the core walls are relatively thin.
A number of alloys exhibit superplasticity and are capable of being subjected to superplastic forming to produce parts of predetermined shapes. Superplasticity is the capability of a material to develop unusually high tensile elongation with reduced tendency toward local necking during deformation. Prior to such superplastic forming, diffusion bonding of the metal workpieces is carried out to bond the workpieces in certain preselected areas, to permit superplastic forming to be carried out in the unbonded areas of the workpieces.
Structures have been successfully produced from a number of titanium-based alloys by the Superplastic Forming/Diffusion Bonding (SPF/DB) process. Such structures are producible because such metals or alloys exhibit the two essential physical properties required for SPF/DB, namely ability to be diffusion bonded and superplasticity.
Diffusion bonding refers to the solid-state, metallurgical joining of surfaces of similar or dissimilar metals by applying heat and pressure for a time duration so as to effect intimate surface contact and cause commingling of atoms at the joining interface.
Examples of metals which can be diffusion bonded and which have superplasticity characteristics include titanium, zirconium, refractory metals, and alloys thereof Aluminum may also be suitable for this purpose since recent developments indicate that aluminum and its alloys can be diffusion bonded, as well as being capable of superplastic forming.
U.S. Pat. No. 3,927,817 discloses a method for fabrication of structures in which metal blanks, preferably diffusion bonding at elevated temperatures and pressures, and then subjected to superplastic forming to form a desired structure. The metal blanks are first treated at selected areas with a stopoff material, such as yttria, boron nitride, graphite, or alumina to prevent bonding at such treated areas during diffusion bonding. During superplastic forming the metal blanks are expanded at the treated (unbonded) areas into contact with shaping members by increasing the internal pressure, preferably with an inert gas, thus forming an expanded structure of a desired shape, essentially in a single operation.
Thus, after the bonds between adjacent metal blanks are formed during diffusion bonding, inert gas pressure, such as argon or helium, is applied to the interior network to superplastically form the unbonded portions of the adjacent metal sheets.
U.S. Pat. No. 4,303,570 is further illustrative of the production of diffusion bonded and superplastically formed structures and techniques employed therein.
When producing truss core panels or sandwich structures, after diffusion bonding and superplastic forming of such structures, distortion or crushing of the core, particularly in cases where the core is relatively thin, e.g., of the order of 0.030 inch thick or less, generally occurs, Although such core distortion is apparently due to some external force, such as the tool itself or atmospheric pressure on the expanded core structure, the actual cause or causes of such core distortion have not been fully identified, except that it occurs during the gas evacuation or the cool-down cycle, following expansion and superplastic forming, due to pressure or thermal gradients. Instantaneous pressure gradients including the negative internal pressure caused by the cooling and contraction of internal gas, may be caused by the inability of the gases remaining in the interior of the panel structure to equalize with the ambient air through the small gas injection tube orifices in the tool. Thermal gradients may be due to the fact that since the steel tool contracts faster than, for example, the titanium alloy panel which has been diffusion bonded and superplastically formed, the tool and the panel must be slightly separated to prevent tool pressure and crushing of the titanium alloy panel core. This is often inconvenient and impractical and may require the introduction of shims or spacers between the upper tool and the upper surface of the core panel.
It is accordingly an object of the present invention to provide a process for producing structural members, particularly core panel structures, by diffusion bonding and superplastic forming, while eliminating core distortion or crushing.
Another object of the invention is the provision of core structures of the above type, wherein the thickness of the core is relatively thin, while avoiding core distortion.
A still further object of the invention is the provision of thin truss core panels by diffusion. bonding and superplastic forming, embodying a simple procedure for eliminating core distortion.